The present invention relates to an angular velocity sensor known as a gyroscopic instrument and more particularly, to a high-performance angular velocity sensor having a tuning-fork construction where two vibrator units containing piezoelectric elements are coupled to each other and a method of fabricating the same.
A conventional gyroscopic inertia navigation system includes mechanical rotor gyros for determining the direction of a moving object, e.g. an airplane or ship.
Such a mechanical gyroscopic system is steady in performance but bulky in size, thus increasing the cost of production and precluding its application to any small-sized pertinent apparatus.
Also, an oscillator-type angular velocity sensor is known for detecting a "Coriolis force" with its detector while it is vibrating but not rotating. Such a sensor commonly employs a piezoelectric or electromagnetic oscillation mechanism.
The detection of an angular velocity in the sensor is implemented by sensing a vibration torque of a frequency equal to that of the mass of a gyro which is not rotating but vibrating at a constant rate. The vibration torque is known as the Coriolis force generated when an angular velocity is involved.
The oscillator-type angular velocity sensor can detect the amplitude of the vibration torque to determine an angular velocity. Particularly, a variety of oscillator-type angular velocity sensors employing piezoelectric elements have been introduced (for example, as depicted in the Proceeding of Japanese Institute of Aviation and Space, Vol. 23, No. 257, pp. 339-350).
One of the conventional tuning-fork structure oscillator-type angular velocity sensors will now be described referring to FIGS. 5 to 7. The arrangement of the angular velocity sensor is best illustrated in FIG. 5 which consists mainly of four piezoelectric bimorphous elements serving as an actuator, a monitor, and a first and a second detectors. The actuator 101 is orthogonally coupled by a joiner 105 to the first detector 103 constituting a first vibrator 109 while the monitor 102 is orthogonally coupled by another joiner 106 to the second detector 104 constituting a second vibrator 110. The first and the second vibrators 109, 110 are coupled to each other by a connector 107 which is supported at a point by a support 108, thus constructing a tuning-fork structure.
When the actuator 101 of piezoelectric bimorphous element is loaded with a sine-wave voltage signal, its inverse piezoelectric effect causes the first vibrator 109 to vibrate. Then, the action of the tuning-fork structure results in vibration of the second vibrator 110.
Accordingly, the monitor 102 of piezoelectric bimorphous element generates a charge on its surface through its piezoelectric action. The charge is proportional to the sine-wave voltage signal applied to the actuator 101. Hence, a constant, continuous action of vibration is developed by controlling the sine-wave voltage signal to the actuator 101 so that the charge generated by the monitor 102 remains uniform in amplitude.
The action of the angular velocity sensor for producing an output corresponding to an angular velocity involved will be explained referring to FIGS. 6 and 7. FIG. 6 is a top view of the angular velocity sensor of FIG. 5. As shown, the turning movement at an angular velocity of .omega. produces a Coriolis force on the first detector 103 which vibrates at a speed of v. The Coriolis force is at a right angle to the speed v and its magnitude is 2mv.omega. (where m is the equivalent mass at the distal end of the first detector 103).
As the first detector 103 vibrates at the speed v, the second detector 104 is responsive to vibrate at -v and a Coriolis force on the second detector 104 is -2mv.omega.. The two detectors 103 and 104 are stressed in opposite directions by their respective Coriolis forces, as shown in FIG. 7 thus producing charges on the surface through their piezoelectric actions.
When the speed v of vibration created by fork oscillation is expressed by: EQU v=a.multidot.sin .omega..0.t
ps where a is the amplitude of the vibration and .omega.o is the period of the vibration, the Coriolis force is: EQU Fc=a.multidot..omega..multidot.sin .omega..0.t
While the angular velocity .omega. is proportional to the vibration amplitude a, the Coriolis force causes either of the two detectors 103 and 104 to deflect in one direction. Hence, the surface charge Q on the detectors 103 and 104 is expressed by: EQU Q.varies.a .multidot..omega..multidot.sin .omega..0.t
When the vibration amplitude a is controlled to a constant, EQU Q.varies..omega..multidot.sin .omega..0.t
As understood, the surface charge Q is found proportional to the angular velocity .omega. and can be converted to a direct current signal through synchronous transaction at .omega.ot.
In theory, if the angular velocity sensor is subjected to a translational movement rather than rotation, its two detectors 103 and 104 produce two charges of the same polarity and their resultant DC signals are suppressed by each other, thereby generating no output.
However, the two signals derived from the unwanted charges are not always canceled to zero because of a symmetrical error and a difference in weight between the two, left and right, prongs of the tuning-fork structure. Conventional angular velocity sensors in which a plurality of piezoelectric bimorphous elements are assembled in a relatively complex manner may not be identical in quality.
For overcoming these disadvantages, best care is taken to assemble the tuning-fork structure to ensure the symmetry and balance of the fork structure. So far, such efforts are found to be unsuccessful and fail to cancel both undesired signals. The two unwanted signals cause the sensor to deteriorate the thermal characteristics and become oversensitive to external interruption or vibration.
Another conventional tuning-fork structure oscillator-type angular velocity sensor has each prong made by twisting a metal base plate to form two, detector and actuator, parts and is known as a bend-type structure sensor which is improved in structural strength. Unfortunately, the metal base plate is bent along off the center line. This creates an undesired stress at the bend causing a displacement or change in the orthogonal relation between the detector and the actuator upon thermal variation. As the result, it is not possible for the angular velocity sensor of this type to maintain the offset output change to a minimum throughout a wide range of temperature.
Furthermore, a modified conventional tuning-fork structure angular velocity sensor is provided in which the metal base plate is replaced with a metal base block for ensuring no change in the orthogonal relation and giving a higher accuracy in performance. The sensor offers better thermal characteristics without changing the orthogonal relation but is more expensive.
The present invention is directed towards eliminating the above disadvantages and its object is to provide an improved angular velocity sensor which has better thermal characteristics and particularly, is less costly.